The objective of this research proposal is the study of the functions involved in progression through the G1 and S-phase of the cell cycle in somatic animal cells, using conditional lethal temperature sensitive cell mutants defective in different steps of cell cycle progression. Another part of this proposal deals with the study of the functions regulating gene transfer in animal cells. Our work is planned along five main lines of research: 1) The study of the ts BHK mutant ts BN-2, which at 39.5 degrees shows interruption of S-phase DNA synthesis and premature chromosome condensation. We plan to identify the primary defect of this mutant and use these cells to study the structure of prematurely condensed chromatin. 2) The isolation and study of additional ts mutants of BHK cells affected in G1 and S-phase progression. 3) Attempt to identify the gene products affected in the cell cycle mutants, using complementation by wild type proteins. 4) The study of gene-transfer in animal cells with particular regard to the state of the transferred gene(s). The isolation of cells containing the polyoma virus genome integrated in a specific cellular gene and the study of the effects of such integration on gene expression.